An electrophotographic image forming apparatus includes a fixing device that applies heat and pressure to a sheet material, such as a sheet of paper, onto which a toner image has been transferred, so as to fix the toner image. There is a fixing device that has a heat-roller fixing system in which a nip section is formed by a fixing roller that has an internal heat source, such as a halogen heater, and by a pressing roller that is in contact with the fixing roller; and the sheet material to which the transferred toner image has not been fixed is conveyed through the nip section so that the unfixed toner image is fixed to the sheet material.
Furthermore, a fixing device that has a belt-fixing system is also used, in which a ring-shaped fixing belt is extended between a heater roller that has an internal halogen heater, or the like, and a fixing roller that is located adjacent to the heater roller; a nip section is formed by the fixing belt and a pressing roller that is in contact with the fixing roller via the fixing belt; and the sheet material to which the transferred toner image has not been fixed is conveyed through the nip section, whereby the unfixed toner image is fixed to the sheet material.
The fixing device that has this fixing-belt system mentioned above offers advantages in that, because of the low heat capacity of the fixing belt, warming-up time can be shortened and power consumption can be suppressed.
In the fixing device, the toner image that adheres to the sheet material is brought into contact with the fixing roller and the fixing belt; therefore, the surfaces of the fixing roller and the fixing belt are coated with a fluorinated resin that provides superior separation performance, and a separation claw is used to separate the sheet material.
The major disadvantage in using the separation claw is in that, as the separation claw is brought into contact with the fixing roller and the fixing belt, the surfaces of the fixing roller and the fixing belt can easily receive a claw mark (claw scar) and, if the fixing roller, or the like, is damaged, a line occurs on the image output to the sheet material.
Usually, in the case of black-and-white image forming apparatuses, a metallic roller whose surface is coated with silicone resin is used as the fixing roller; therefore, even if the separation claw is brought into contact with the fixing roller, it is hard for the fixing roller to be damaged and the operating life can be longer. Color image forming apparatuses use silicone rubber coated with fluorine for the surface layer (generally a PFA tube of several dozen micrometers is used) or use silicone rubber with oil applied to its surface, whereby improving the colors.
However, a fixing roller that has the above configuration has a problem in that the surface layer is soft and easily becomes damaged. If the surface layer becomes damaged, a linear scar occurs on the fixed image. Therefore, nowadays, many color image forming apparatuses do not use a contact unit, such as a separation claw, and most of them perform non-contact separation.
For non-contact separation, if the adhesion between the toner and the fixing roller is strong, the fixed sheet of paper easily becomes wrapped around the roller and jammed; therefore, a wrapping jam easily occurs. Particularly with color image formation, a number of toner layers are formed and, because of the increased adhesion, a wrapping jam easily occurs.
Nowadays, color image forming apparatuses mainly use the following systems (1) to (3) for sheet separation:
(1) A non-contact separation plate system in which a small gap (about 0.2 mm to 1.0 mm) is provided between the fixing roller and the fixing belt; and separation plates are used that are arranged parallel to the fixing roller and the fixing belt in the longitudinal direction and the cross direction.
(2) A non-contact separation claw system in which a small gap (about 0.2 mm to 1.0 mm) is provided between the fixing roller and the fixing belt; and separation claws are used that are arranged at a predetermined interval.
(3) A self-stripping system in which a sheet of paper is naturally separated due to the rigidity of the paper and the elasticity of the curved portions of the fixing roller and the fixing belt.
In any of the above-described systems (1) to (3), however, a clearance is provided for a sheet guide plate guiding to the fixing outlet; therefore, when a thin sheet of paper or a sheet material that has a small margin at its end is delivered, or when a sheet material on which a solid image such as a picture is formed is delivered, the sheet material passes through the clearance while it adheres to the fixing roller or the fixing belt. Thus, a sheet wrapping jam may occur, or a jam may occur because the sheet hits the separation plate or the separation claw.
In order to ensure non-contact separation of a sheet material, a technology for spraying compressed gas, such as air, at the sheet separation position has been disclosed and is used (see, e.g., Patent Literature 1, Patent Literature 2, Patent Literature 3, Patent Literature 4, Patent Literature 5, and Patent Literature 6, which will be described below). According to the above, compressed air is sprayed at the gap between the fixed toner image on the sheet of paper and the fixing member so that the sheet is forcibly separated from the fixing member.
In recent years, it has become typical with color electrophotographic toner that when a large amount of toner is attached to the sheet material, it is required to increase the accuracy with which compressed air is sprayed in order to ensure removal of the sheet material from the fixing sheet. Therefore, it is necessary to accurately maintain the positional relation between the nozzle member that sprays compressed air and the surface of the fixing member. Specifically, the gap between the end of the nozzle member and the fixing member is kept constant regardless of the rotary movement of the fixing member and thermal expansion of the fixing member.
Patent Literature 7, which will be described below, discloses a technology for dealing with the oscillation of the surface of a fixing member that is a rotary body or a change in the diameter of the fixing member due to thermal expansion so as to accurately maintain the gap between the non-contact separation claw and the surface of the fixing member. A positioning plate is brought into contact with and is pressed against the surface of the fixing member so that the positioning plate oscillates such that the gap between the end of the non-contact separation claw and the fixing member is kept constant.    [Patent Literature 1] Japanese Patent Application Laid-open No. S51-104350    [Patent Literature 2] Japanese Patent No. 2876217    [Patent Literature 3] Japanese Patent Application Laid-open No. H11-334191    [Patent Literature 4] Japanese Patent Application Laid-open No. 2007-189015    [Patent Literature 5] Japanese Patent Application Laid-open No. 2007-240920    [Patent Literature 6] Japanese Patent Application Laid-open No. 2008-102408    [Patent Literature 7] Japanese Patent Application Laid-open No. 2009-31759
However, the technology disclosed in Patent Literature 7 can not be applied to the fixing device that includes a nozzle member without making changes to the technology.
There is a need to provide a sheet separating device that is capable of accurately maintaining the positional relation between the nozzle member and the fixing member, wherein the nozzle member sprays compressed gas at the gap between the fixed sheet material and the fixing member so as to separate the sheet, and to provide a fixing device and an image forming apparatus.